1. Field of the Invention
The invention relates to the mixing of gases and liquids. More particularly, it relates to the oxidation of organic chemicals with pure or nearly pure oxygen.
2. Description of the Prior Art
In organic chemical oxidation reactions in which the oxidation products or byproducts are not precipitated in the reactor as solid materials, direct contact cooling, as by the use of cooling coils, is conveniently used to remove the heat of reaction. In three phase systems in which some portion of the reaction mixture is a precipitating solid phase, however, the precipitation of solids on the heat transfer surfaces can rapidly reduce the transfer capacity of said surfaces. In addition, the surface area of the heat transfer surfaces for the removal of heat in highly exothermic organic chemical oxidation reactions can be quite large relative to the reactor volume.
Most commercial liquid phase oxidations of organic chemicals are carried out using air as a convenient source of oxygen. In such oxidation processes, the inert nitrogen component of the feed air strips off a portion of the volatile components in the reaction mixture. The cooling effect due to the latent heat removal through such evaporation thus balances the exothermic heat of the oxidation reaction. For a given amount of excess air, or inert gas flow through the oxidation reactor, a relationship exists between the temperature of the oxidation reactor and the operating pressure at which the oxidation reaction is carried out. This relationship between the reaction temperature and pressure depends on the composition of the reaction mixture and the volume of excess gas employed. Air based evaporative cooling processes generally require relatively high pressure and temperature conditions for any given organic chemical oxidation.
The feed air passing to the reactor must be compressed to a pressure somewhat above the reactor operating pressure before it is blown into the reactor through a pipe or other submerged sparger. As the air bubbles are dispersed and circulated throughout the liquid phase, the oxygen concentration in said bubbles decreases as the oxygen dissolves and reacts with the organic chemical in the liquid phase. The air bubbles disengage from the liquid phase and collect at the top of the reactor to form a continuous gas phase. This overhead gas phase constitutes a waste gas that must be vented in order to provide room fresh feed air while maintaining adequate gas hold-up to promote the desired transfer of oxygen from the feed air to the organic chemical-containing liquid phase.
To avoid the possibility of fire or explosion, the oxygen concentration in the overhead gas space at the top of the reactor must be maintained below the flammable limit. For this purpose, the oxygen concentration must be maintained at less than 8-9% by volume. More typically, the oxygen concentration in the gas space is maintained below 5% by volume to provide a safe margin below the flammable limit. Thus, in a well stirred tank reactor, the average concentration of undissolved oxygen in the circulating air bubbles must be below 5% in order to insure that the average concentration of oxygen in the gas that collects in the headspace of the reactor is nonflammable.
The oxygen concentration in the gas space is a function of the rate at which feed air is fed to the reactor and the rate of consumption of oxygen from the feed air by reaction with the organic chemical being oxidized. For most liquid phase oxidation reactions, the overall rate of oxygen consumption is determined by the rate at which the oxygen in the gas phase, i.e. gas bubbles, can transfer into the liquid phase. Since the oxygen transfer rate is proportional to the oxygen partial pressure in the gas phase, which is proportional to the volumetric fraction of the oxygen in the gas phase, the 5% oxygen restriction in the gas phase, as referred to above, effectively limits the oxygen mass transfer rate, and therefore, the overall organic chemical oxidation rate.
As air bubbles circulate within the reactor, solvent, water, volatile organic chemicals (VOC's) and byproduct gases, such as CO.sub.2 and CO, collect in the continuous overhead gas space, and are vented from the reactor. The total amount of volatile species that leave the reactor with the inert vent gas is proportional to the total gas throughput, which is proportional to the air feed rate.
In the United States of America, applicable federal, state and local air quality standards that pertain to a particular production facility determine the degree to which these volatile species must be removed from the vent gas before being released to the atmosphere. Solvent materials are typically valuable constituents of the oxidation processes, so they are usually condensed and recycled to the reactor. Residual organic compounds are usually stripped from the inert vent gas, thereby producing a liquid waste stream from the stripper bottoms. Some vent gas treatment systems may also include COx abatement systems as needed to meet air quality standards. Since the total amount of material that must be removed from the vent gas is proportional to the air feed rate to the reactor, the size of the vent gas treatment equipment and the amount of waste that is generated in the oxidation process, is similarly proportional to the air feed rate.
Pure or nearly pure oxygen offers many potential advantages in such organic chemical oxidation reactions. However, the safe, efficient addition of pure oxygen feed into oxidation systems requires the use of special precautions because of the potential for fires or explosions. The Litz et al. patent, U.S. Pat. No. 4,800,480, discloses a highly desirable Liquid Oxidation Reactor (LOR) system for use in place of a conventional reactor system, which is not suitable or is inefficient when used with feed oxygen instead of feed air. The LOR system enables gas bubbles to be recirculated with a recirculating flow of a portion of the organic chemical liquid composition, separated from the overhead gas space, in order to enhance the oxygen use efficiency, while avoiding the loss of appreciable amounts of gas to the overhead gas space. As the gas bubbles are recirculated, and as the oxygen is transferred to the liquid phase, the concentration of oxygen in the gas bubbles decreases. The mass transfer advantage offered by the use of pure oxygen is, therefore, diminished.
For organic chemical oxidations that react very rapidly, the oxygen use efficiency is naturally very high. Thus, a high percentage of the oxygen is consumed on the first pass through the impeller means used in the LOR system, and the mass transfer advantage is greatly diminished in subsequent passes through the impeller means. For such systems, the recirculation of the gas bubbles is undesirable. In addition, because of the nature of the downward pumping impeller and surrounding draft tube used in the LOR system as described by Litz, et al., high volumes of gas in the draft tube can cause the mixer device to cavitate. If such cavitation occurs, the impeller can no longer pump liquid or break up and disperse oxygen in the form of fine bubbles in the recirculating body of organic chemical liquid. If it were desired to employ evaporative cooling in place of commonly used direct contact cooling means, the presence of more volatile or vapor in the reactor would be required than for direct contact cooling processes. If large amounts of vapor were to be recirculated into the draft tube, however, undesired cavitation would likely occur and disrupt the desired mixing of the pure oxygen feed and the liquid being oxidized. As evaporative cooling is advantageous in that it eliminates the problems encountered in the use of direct contact heat exchange surfaces, a modification of the LOR impeller/draft tube system is needed in order to reduce the amount of recirculated gas in the reactor, so as to enhance the overall performance of LOR systems as used in evaporatively cooled oxidation processes.
It is an object of the present invention, therefore, to provide a process and system for the oxidation of organic chemicals, using evaporative cooling of the reaction mixture to eliminate the problems associated with the use of direct contact heat exchange surfaces.
It is another object of the invention to provide an LOR process and system using evaporative cooling and pure or nearly pure oxygen for the oxidation of organic liquids.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.